Abstract

In the granite-gneiss basement of the Zou basin, drinking water access is mainly provided by fracture aquifers. In order to facilitate the borehole drilling, this study, on one hand, established the fracture map and on the other hand evaluated the groundwater potential in this part of Benin. First, Sobel directional filters are used to extract a representation of the fracture network. From these data, three majority directions are identified: N180-N30, N90- N110 and N60-N70. Other minority groups, in particular, N130-N140, also affect these crystalline and crystallophyllian formations. The fracture lengths are concentrated in a range of 1 to 2 km (65 %) and the extreme values lower than 1 km and greater than 2 km are respectively 12.7 % and 22 %. This set of direction reveals an average fracture density of 2.4 km.km-2 with areas such as the South of Iroukou having low values (0.8 ± 0.9 km.km-2 ) and high values in Dassa (5.2 ± 0.9 km.km-2 ). These results are useful for drilling because more than 60% of boreholes located within 200 m of an identified fracture is positive. In addition, other maps such as alteration thickness, lithology, drainage, rainfall, and slope are confronted with fracture density using multicriteria decision analysis. Based on the contribution of each map to groundwater availability, the weights are 0.158, 0.074, 0.126, 0.302, 0.065 and 0.275, respectively. It turns out after spatial analysis that the basin is characterized by a very low (9.2 %) to low (18 %) potential towards Djidja, Gangan and Glazoué, moderate (24.7 %) in the Zoudji and Yagbo and good (27.5 %) to very good (20.6 %) around Savalou. The water potential of these blocks of aquifers can change significantly even at the local scale. Despite the constraints, these resources are essential to achieving the Sustainable Development Goal 6 in the basin, of course after a thorough study of the aquifers in order to guarantee drinking water supply of the populations and the durability of the boreholes.